In the iron industry, as in other industries making efforts these days for preventing pollution, the dust generated by blast furnaces and steel producing furnaces (e.g. converters, open-hearth furnaces such as those producing steel from pig iron and/or scrap iron: hereinafter referred to as "steel furnace") has been collected so it will not be diffused or scattered, in order to eliminate the pollution problem. Various dust collecting means such as electric precipitators, venturi-scrubbers, bag filters or the like have been used in association with the respective furnaces. However, the enormous amount of dust thus collected may likely cause secondary pollution problems depending on the manner of disposal or dumping.
Also it has been pointed out that the dust thus collected contains a relatively high proportion of ferrous material and therefore the dumping of the dust is a waste of reusable resources.
Accordingly it has also been proposed to manufacture reduced pellets from the dust by granulating the same. One of the ways for producing such pellets is to mix and knead blast furnace dust and steel furnace dust wherein the latter serves as binding agent to produce very dense material of high strength.
However, in this prior art method, it is necessary to mix the blast furnace dust and steel furnace dust uniformly before adding water (about 10% by weight) to proceed kneading. However, it is difficult to mix these two kinds of dust uniformly. Also, such mixing requires a large mixing apparatus or an air blender and the secondary dust generated by use of these mixing apparatuses creates another pollution problem.
In a steel mill, or a iron foundry, dust is generated by, for example, blast furnaces, open-hearth furnaces, converters, yard thickeners, classifiers, sintering apparatuses, monitors in the factory buildings, dust collecting apparatuses and so on and the physical and chemical properties of the dust vary from place to place depending on where it is generated. Therefore, if it is desired to process all the dust generated in the iron foundry into reduced pellets, it is essential to mix the dust uniformly and this has heretofore been quite difficult.
Also, there are dust collectors of wet type which discharge the dust in a slurry state. The treatment of such slurry becomes a problem, particularly in the case of blast furnace dust and the concentration thereof is low, since blast furnace dust contains many coarse particles and, thus, the sedimentation rate of the blast furnace dust in a basin is high and compressing and packing or condensing effect in the lower part of the basin may make it difficult to transport the dust to the next stage through conduit means.
As a whole, it has been found that, in order to uniformly mix the dust discharged from several places in the iron foundry, it is necessary to control the feeding or supplying condition of several kinds of dust. In other words, in order to produce reduced pellets of high quality, it is necessary to feed mixed dust while maintaining its chemical and physical composition and properties constant; otherwise a product of high quality may not be obtained.